1. Field of the Invention
The present invention generally relates to a demodulating apparatus. More specifically, the present invention is directed to a demodulating apparatus employed in a transmission system in which digital data is modulated by a phase modulating method and the phase-modulated digital data is transmitted.
2. Description of the Prior Art
A typical circuit arrangement of this sort of a conventional demodulating apparatus is shown in FIG. 1. In this conventional demodulating apparatus, PSK-modulated digital signal which is modulated by way of the PSK (phase shift keying) modulating method is received, and then this PSK-modulated digital signal is demodulated.
In this case, it is assumed that this demodulation system is realized by employing a single demodulator for demodulating transmission signals having respective different transmission speeds (modulation speeds).
In this demodulating apparatus, the modulated signal entered from the input terminal is separated into two series of the modulated signals by a signal separator 1, which will then be supplied to two detectors 2 and 3. In these detectors 2 and 3, these modulated signals are multiplied by local oscillation signals derived from a local oscillator 5. One local oscillation signal is phase-shifted by a phase shifter 4. These input modulated signals are quasi-synchronizing-detected to be converted into baseband signals. At this time, the oscillation signal from the local oscillator 5 has the same frequency as that of the carrier wave contained in the modulated signal. Since one oscillation signal is phase-shifted by 90.degree. in the phase shifter 4, a pair of local oscillation signals which are perpendicular to each other are produced and then supplied to the corresponding detectors 2 and 3, respectively.
Two series of the baseband signals which have been quasi-synchronizing-detected are entered into groups of low-pass filters (LPFs) 6-1 to 6-n and 7-1 to 7-n, respectively. These low-pass filters 6-1 to 6-n and 7-1 to 7-n have characteristics corresponding to the transmission speeds of the received modulated signals in order to remove unwanted signals (spurious signals) from the respective baseband signals. Accordingly, a return noise can be suppressed and a poststage circuit for these low-pass filters can be prevented from being saturated.
The filter outputs from the LPFs 6-1 to 6-n are selectively derived by a selector 8, and similarly, the filter outputs from the LPFs 7-1 to 7-n are selected by a selector 9 in accordance with the transmission speed, respectively. To achieve this signal selection, these selectors 8 and 9 are controlled in response to externally supplied switching signals. The analog signals outputted from the respective selectors 8 and 9 are converted into digital signals by A/D converters 10 and 11 in response to a sampling clock outputted from a digital demodulator 12, and thereafter demodulated by this digital demodulator 12.
It should be noted that the LPFs 6-1 to 6-n and 7-1 to 7-n have such functions to avoid occurrences of the return noise in the digital signal process effected in the digital demodulator 12 provided at the poststage, and also saturation of the poststage circuit caused by the adjacent waves. Since-the pass bandwidths of the low-pass filters are determined on the basis of the transmission speeds and the levels of the adjacent waves, it is required that a demodulating apparatus for demodulating the data with the different transmission speeds is equipped with a plurality of low-pass filters 6-1 to 6-n and 7-1 to 7-n capable of accepting all of these different transmission speeds, and also with the selectors 8 and 9 for properly switching these low-pass filters.
Now, a description is made of a selection of the LPFs 6-1 to 6-n and 7-1 to 7-n in the demodulating apparatus of FIG. 1. It should also be noted that the adverse influences caused by the adjacent waves are negligible in this case. It is now assumed that there are three sorts of transmission speeds of the signals demodulated by this PSK demodulating circuit, i.e., 2 Mbps, 10 Mbps, and 20 Mps (each of symbol speeds being 1 Mbps, 5 Mbps, and 10 Mbps), and further the maximum sampling clock speed is 20 MHz due to hardware restrictions. Under such a condition, since the maximum sampling clock speed (frequency) is limited to 20 MHz, the number of samples per 1 symbol is limited to "2".
Also, the low-pass filters are determined based on the specifications required by the demodulator, and thus determined based upon the below mentioned conditions in this case: EQU Fc=(0.7.times.symbol speed)Hz to (symbol speed) Hz - - - (1)
Note: Symbol Fc shown in equation (1) indicates the cut off frequency of LPF.
As apparent from the foregoing descriptions, the following three different sorts of LPFs corresponding to each of the symbol speeds are required. The cut off frequency Fc of the LPF selected to the symbol speed 1 Mbps is 0.7 MHz to 1 MHz; the cut off frequency Fc of the LPF selected to the symbol speed 5 Mbps is 3.5 MHz to 5 MHz, and further the cut off frequency Fc of the LPF selected to the symbol speed 10 Mbps is 7.0 MHz to 10 MHz. That is, three different sorts of LPFs 6-1 to 6-3, and 7-1 to 7-3 must be previously employed in the conventional demodulating apparatus.
In the conventional demodulating apparatus shown in FIG. 1, the sampling clock produced from the digital demodulator 12 is constant (namely, number of samples per 1 symbol is constant), and therefore, when the transmission speed is increased, it is not possible to increase the number of samples due to the hardware restrictions. As a consequence, a plurality of low-pass filters must be employed in accordance with the transmission speeds and must be selectively used in order to eliminate the return noise. Thus, a plurality of LPFs and the selectors are necessarily required, resulting in increases of the hardware.